import java.util.Arrays;

public class MyStack {
    public int[] elem;
    public int usedSize;

    public MyStack(){
        this.elem = new int[10];
    }

    public void push(int val){
        if(isFull()){
            this.elem = Arrays.copyOf(elem,2*elem.length);
        }
        elem[usedSize++] = val;
    }

    public boolean isFull(){
        return usedSize == elem.length;
    }

    public int pop(){
        if(isEmpty()){
            throw new RuntimeException();
        }
        int val = elem[usedSize-1];
        usedSize--;
        return val;
    }
    //获取栈顶元素 但是不删除
    public int peek(){
        if (isEmpty()){
            throw new RuntimeException();
        }
        return elem[usedSize-1];
    }

    public boolean isEmpty(){
        return usedSize == 0;
    }
}
